1. Field of Invention
The invention relates to a map display system for drawing a polygon map by determining a unit of a polygon map to be displayed in the background of a road map, and a memory medium.
2. Description of Related Art
In prior map displays for a navigation system, polygon maps showing boundaries of administrative divisions have been displayed in the background of a road map. Since data for these polygon maps have a predetermined administrative division unit and associated color stored, when a map is displayed on any scale, polygon maps are displayed in the same unit and color. Therefore, for example, in a large area map with boundaries for the administrative divisions of Japan (other countries having different administrative divisions), the prefectures for Tokyo, Hokkaido, Osaka, and Kyoto are displayed. However, when a detailed map is displayed without showing the boundaries of the administrative divisions, the background color of the road map is displayed in the same unified color for all the scales.
If polygon map data is stored on the basis of a small administrative division (such as “Oaza”—a large section of village), the unit of the polygon map is too small and the entire map becomes difficult to view when a large area map is displayed. On the other hand, if polygon map data is stored on the basis of a large unit (such as Prefecture), a screen is filled with one unified color and the boundaries of the administrative divisions are not displayed when a detailed map is displayed. Thus, information on a vehicle present position is difficult to read from the map information. It is also difficult to distinguish between boundaries and road information because if the boundaries of administrative divisions are simply displayed with lines, the boundaries can be mistaken for roads.